1. Field of the Invention
The present invention relates to an electrolyte for a lithium secondary battery and a lithium secondary battery including the same, and more particularly, to an electrolyte for a lithium secondary battery and a lithium secondary battery including the same which has excellent swelling characteristics and discharge capacity at a low temperature.
2. Description of the Related Art
With recent developments in the high-tech electronic industry, electronic appliances can be smaller in size and more lightweight, and thus the use of portable electronic devices is increasing. Due to the rising demand for batteries with high energy density to power such portable electronic devices, research on lithium secondary batteries has progressed. Lithium-transition metal oxide is used as a positive electrode active material, and carbon (crystalline or amorphous) or a carbon complex is used as a negative electrode active material. The active material is applied to a current collector to a predetermined thickness and length, or formed in a film shape to be wound with a separator, which is an insulator, or stacked, so as to form a battery. Subsequently, this is put into a metal can or a similar reservoir and an electrolyte is injected thereinto, and thus a rectangular box-shaped secondary battery is produced.
The electrolyte includes a lithium salt and an organic solvent. As the organic solvent, solvents of two to five-component systems, which are comprised of cyclic carbonate such as ethylene carbonate or propylene carbonate, and linear carbonate such as dimethyl carbonate, ethyl methyl carbonate or diethyl carbonate, has been used. However, these solvents have a problem in which a swelling phenomenon excessively occurs at a high temperature. The swelling phenomenon refers to a phenomenon in which the center of a specific surface is transformed, for example, a battery is swollen in a specific direction due to a gas generated by decomposition of an electrolyte in the battery at a high temperature. This swelling phenomenon still creates a problem in a lithium polymer battery, although not as severe compared to a lithium ion battery, when a nickel-containing active material (for example, LiNiMO2, M=Co, Mn, Al, P, Fe, and Mg, and comprised of at least one of the aforementioned elements) is used as a positive electrode active material.
Recently, there have been many trials on a solvent using γ-butyrolactone having a high boiling point and a high dielectric constant to suppress the swelling phenomenon.
Here, the solvent including γ-butyrolactone has been commonly mixed with ethylene carbonate, however, this has problems of high viscosity and wettability to a separator. Thus, other solvents with a low boiling point and a low dielectric constant have been mixed to be used as alternatives. However, the alternative solvents still have the problem of the swelling phenomenon at a high temperature.